This invention relates generally to films and containers made from high density polyethylene. More specifically this invention is directed toward films and containers having low water vapor transmission rates, while maintaining better physical properties than films or containers fabricated with materials having similar water vapor transmission rates.
Polyolefins have been used as packaging materials for several decades. Among the properties provided by polyolefin packaging materials are durability, case of machineability, protection of packaged contents, and printability.
In the segment of the market where polyolefin based packaged products are used for protection, there are several types of protection sought depending upon the products packaged, their desired shelf life, and the environment to which the packages and their contents are to be subjected. One of the key elements that a polyolefin protective package (or a package including a polyolefin) can provide is a resistance to moisture or water vapor transmission. This resistance can either keep moisture away from packaged contents or alternatively keep moisture in packaged contents. In either case, generally, the higher the water vapor transmission resistance per unit of film or container thickness, the more economical the package.
One of the most common polyolefin polymers valued for its moisture or water vapor transmission rate (WVTR) is high density polyethylene (HDPE). Generally HDPEs are those which have densities at or above about 0.940 g/cc. Generally, the higher the density, the better a resin""s WVTR for a given package thickness. However, as with many such physical properties, there are tradeoffs, because as the resin density is raised to improve or decrease the WVTR (leading to a lower water vapor transmission value), at least some physical properties of a film or container manufactured from the higher density materials will generally be poorer than those of a film or container produced by lower density materials. These poorer physical properties can manifest themselves in, for instance, increased splittiness or lack of balance in tear properties of a film, or greater tendency for crack propagation in a container. Accordingly, package designers and converters would generally like to have improved physical properties at the same or similar WVTR, or in the alternative, improved WVTR at similar densities. Alternatively, these designers or converters could, by utilizing films or containers made from the resins of an embodiment of the present invention, have lower (better) WVTR, than from resins previously obtainable (with both resins having the same or similar densities).
Applications for which such high density products can be used include films for cookie and cracker packaging. In these particular cases, the object of the package is not only to hold the contents, but also to provide resistance to moisture vapor transmission (from the environment to inside the package) which would diminish the shelf life of the contained cookies, crackers, or the like, where the shelf life is determined by the time it takes the products to pick up sufficient moisture to render them stale. In cookie and cracker packaging applications for example the general object of the barrier layer is to substantially keep moisture out or to slow its ingress.
In other types of packaging applications, pouches and pails, where the contained material is hygroscopic, for instance, detergent powders, the same function of slowing ingress of moisture is generally performed. On the other hand, there are also packaged products which contain moisture and which if reduced in moisture-content through, for instance, evaporation to the atmosphere, would destroy or impair the function of the contained products. In all of these cases the package is depended upon to prevent the transmission of water vapor in one direction or the other.
Copending application U.S. Ser. No. 08/093,501 discloses metallocene catalyst systems that can be used to produce polymers having not only excellent strength, sealing, and optical properties, but having superior water vapor transmission rates. The polymers are disclosed to have use in the packaging industry. A film is disclosed having at least one layer having a density less than about 0.935 g/cm3, a Mw/Mn less than about 3, a CDBI greater than about 80%. The layer includes a resin having a density about 0.90 g/cm3 and a WVTR of less than about 2.25 g/mil/100 in2/day.
In the past, higher density polyethylenes were needed to achieve a certain WVTR, alternatively a second layer could be used to provide other properties, often relatively poor in HDPE; for instance, physical properties such as tear resistance, and/or mechanical properties such as heat seal, not generally available from such a HDPE. However, such combinations may result in added costs and may effect other important properties necessary to the packaging industry. Therefore, a need exists for a barrier polymer from which a film or container can be fabricated such that the fabricated article will have relatively low water vapor transmission rates combined with improved physical properties.
It has been discovered that certain metallocene catalyst systems can be used to produce polyethylenes which when converted into films, sheets, or containers having not only excellent physical properties, such as balanced tear resistance and higher dart drop impact, but also having superior water vapor transmission rates as well. These polymers or barrier polymers of the invention are particularly well suited for use in the packaging industry, specifically in those applications in which the combination relatively low water vapor transmission rates and improved physical properties are desirable.
In certain embodiments of my invention a polyethylene resin comprises at least one layer of a film or container or is a constituent of such layer. This layer has a density in the range of from about 0.935 to about 0.965 g/cm3, a Mw/Mn less than about 3, a Mz/Mw less than about 2.5, a Mz+1/Mw less than about 4, and an article made using the resin has a water vapor transmission rate up to about 0.54 g/mil/100 in2/day, preferably up to about 0.4 g/mil/100 in2/day; the resin will elute volatiles including  less than C20 (determined on GC/MS chromatography) of less than about 100 wppm; and a hexadecene volatiles level of less than 10 wppm. The film or container will have either a single layer or multilayer construction and the resin can be coextruded, laminated or blended with other materials.